In rotary and/or percussion earth drilling apparatus, it is common to mount a drill motor on a cradle which can reciprocate on a vertical drill tower, the cradle and drill motor being moved upwardly or downwardly on the tower by a chain which passes around sprockets at opposite ends of the tower and is driven by a suitable drive motor. In drilling apparatus of this type, shock loads and vibration can cause premature failure in the length of chain above the drill-carrying cradle when withdrawing a drill string from the earth. A break in the upper reach of the chain is most likely to occur since during normal drilling operations the drill motor is being pulled downwardly by the chain and the upper reach of the chain is at a relatively low tension; whereas during withdrawal of the drill string, the chain is under relatively greater tension. If the chain should break during the withdrawal process, the cradle and drill may fall, causing injury to operating personnel.